For most people sibling relations last longer than any other type of relations throughout their lives. There are four main areas of research on siblings - the evolvement of their relationship across the life span; connections between sibling relations and other relationships within the family; influence of siblings on the individual development and reasons for differences among siblings. Sibling relationships start even earlier than the arrival of the second child, when parents discuss the expected baby. After the birth, siblings develop complex relationships that can be warm and supportive or hostile and conflicting.
Research shows that including the firstborn child in the care of the baby and treating the baby as a person like the older child can help form a positive sibling relationship. As siblings grow up social support becomes an increasingly important function of their relationship. Childhood rivalries, however, are also likely to reappear when siblings communicate extensively as adults. Although parents are considered to be the most important figures in the life of children siblings play a significant role as well. Older siblings can be playmates, teachers or role models and they can provide advice, therapy or friendship. Common roles for younger siblings include those of advice seekers but they can be competitors too.
Another factor of importance to the lives of siblings is the marital relationship between their parents. Research has shown that conflicts between parents can affect the ability of siblings to develop a positive relationship. On the other hand, hardships like a divorce or death of a parent may bring siblings together and make them bond more closely to each other. Furthermore, research into step-families has demonstrated that divorce and remarriage often have a negative effect on sibling relationships.
Sibling relationships also have an important influence on individual development and general family functioning. Siblings can affect each other as well as other family members by demonstrating positive behavior or by acting in a hostile way. Studies among ethnically diverse families indicate that siblings tend to play an increasingly important role in family life and individual development. A sibling's illness or disability has also been proven to produce negative effects on individual development and sibling relationships.
There are numerous examples of substantial differences in the behavior of siblings who were raised in the same family. The debate over the reasons for such differences goes in two main directions: the genetic structure of each person called nature and the aggregate of shared family experiences like parenting and other social behavior know as nurture. Comparisons between twins, full siblings and adoptive siblings are used by behavior genetics to illustrate individual similarities and differences in personality and behavior. Identical twins (monozygotic or conceived from one egg and one sperm) are easily perceived as similar since they share 100 percent of their genetic material. Therefore differences in their behavior should be the result of environmental influences like relations with parents and friends. However, since identical twins arrive in the family simultaneously, it may be presumed that their environment would be similar in many aspects. On the other hand, despite the fact that they also arrive at the same time, fraternal twins (dizygotic or conceived from two eggs and two sperm) share only about 50 percent of their genes and may not be necessarily the same sex. In this case there are fewer similarities in terms of both genetic structure and social environment, so differences in character and behavior are more likely to appear than with identical twins.
Full siblings, who have the same biological parents, not only share just about 50 percent of their genetic material, but also arrive in the family at different times. Here differences in development may be substantial although the siblings share more or less the same family experiences. With half siblings, who share only one parent, differences may be even bigger due to the fact that they share less genetic material. The fact that siblings in the same family might be so different from each other can be further explained by differential parental treatment. The need for a differentiated approach to siblings in relation to their individual characteristics like age, sex or temperament has been widely recognized. If the parents have favorites, however, this can be perceived as unfair treatment by the neglected siblings and may lead to the development of negative attitudes. Research has indicated that differential parental treatment may not only lead to worsening sibling relations and behavior problems but it can also cause conflicts in the overall family relations as well.